The font and font background color dropdowns contain the “Document colors” section. It lists the colors already used in the document for the users to be able to easily reuse them (for consistency purposes).

Passing an empty value will remove the font background color from the selection:

editor.execute( 'fontBackgroundColor' );

We recommend using the official CKEditor 5 inspector for development and debugging. It will give you tons of useful information about the state of the editor such as internal data structures, selection, commands, and many more.